Preheat oven to 350°F Place sheet of phyllo dough long-side facing you on a parchment paper-lined baking sheet. Spray with butter-flavored cooking spray. Fold phyllo lengthwise in thirds, for a narrow sheet, about 18 inches long. Spray again.
Stir 1 1/2 teaspoons sugar and 1/4 teaspoon cocoa together in cup. Sprinkle sugar mixture over phyllo. Cut phyllo crosswise into 6 equal strips using sharp knife. Spread out on baking sheet. Bake in oven until crisp and lightly browned, 8 minutes. Cool.
Meanwhile, scald milk (heat to just below boiling point) in a small pot. Add remaining sugar and stir to dissolve.
Combine remaining cocoa and cornstarch in cup. Pour in enough hot milk to make a paste. Stir and return to pot. Cook over low heat, stirring constantly, until thick. Spoon into bowl and cool completely.
To assemble, fold whipped topping into chocolate mixture. Spread 2 heaping tablespoons mousse mixture over each phyllo sheet. Top each with banana slices. Place 1 phyllo sheet on dessert plate. Add 2 more layers. Repeat to make second serving. Yields 1 per serving.